Holographic microfabrication and characterization system for soft matter and biological systems

ABSTRACT

A method and system for micromanipulation of objects of any shape. The method and system creates various forms of holographic optical traps for a variety of commercial purposes. Some alternate forms of traps include a dark form of optical traps, optical vortices with different helical winding numbers and optical traps with variable phase profiles imprinted thereon.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims the benefit under 35 USE 119(3) of U.S.Application No. 60/857,348, filed Nov. 7, 2006, incorporated herein byreference in its entirety.

The United States Government has certain rights in this inventionpursuant to grants from the National Science Foundation by Grant NumbersDMR-0451589 and DBI-0629584.

This invention is directed to a fully integrated holographicmicrofabrication system and method. More particularly, the invention isdirected to an optical system for assembling and dynamically controllingcomplex three dimensional structures from objects suspended in a fluid,the objects being, for example, colloidal particles, macromolecules,nanoclusters, nanowires and biological materials, such as cells. Suchobjects can be of any size and shape which are readily assembled andmanipulated for a selected commercial purpose.

BACKGROUND OF THE INVENTION

The use of optical traps or tweezers has undergone substantialdevelopment over recent years. This technique can manipulate matter,including very small objects and small portions of larger objects, withgreat precision. Recent progress has resulted in the ability to createlarge arrangements of optical traps to perform simultaneously many tasksat various spatial locations. These traps can also be individuallyspecified as to trapping strength, optical character and size, given theneeds of the situation. In view of all these degrees of freedom,however, little has been accomplished in terms of complex commercialapplications.

SUMMARY OF THE INVENTION

One Three dimensional assembly, micromanipulation and dynamicconfiguring of objects is accomplished by use of computer generatedholograms which can trap objects, exert precision force at selectedsystem locations and assemble complex arrangements of objects in anyselected three dimensional configuration, including extensive stacks ofobjects. Collectively the assembly of optical traps can executeprocessing and manufacturing protocols for a wide variety of commercialpurposes. This system can carry out such manufacturing steps asassembling three dimensional functional structures from various buildingblocks, such as microscopic fluid-borne objects (colloidal particles,e.g.), macromolecules, nanoclusters, nanowires and various biologicalmedia, such as biological cells. The system can carry out assembly,processing, testing and inspection of the assembled particle array or anobject, execute chemical processing steps, as well as perform mechanicaland optical processing using a selectable range of light wavelengths,including white light to perform these functions. Further, the systemcan be used as a sensor or probe for optical, electrical, chemical,biological and force gradient properties. In addition the system employsa holographically focused microscope with each image itself being ahologram, incorporating volumetric data and in effect is threedimensional versus conventional holographic microscopy where images aretwo dimensional.

Various aspects of the invention are described hereinafter, and theseand other improvements are described in detail hereinafter, includingthe drawings described in the following section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a holographic optical trappingsystem;

FIG. 2A illustrates a computer generated hologram (CGH); FIG. 2Billustrates a light array at an intermediate focal plane; and FIG. 2Cillustrates a resulting optical trap array;

FIG. 3A illustrates an assembly of 173 colloidal silica spheres in aquasicrystal layer array; FIG. 3B illustrates the trapped particlestranslated into a desired three dimensional configuration; FIG. 3C showsreduction of the scale to create an optically dense material which canthen be gelled; and FIG. 3D shows a laser diffraction pattern at 633 nmwavelength which shows 10 fold diffraction peaks;

FIG. 4A illustrates a helical phase mask transformation of a TEM₀₀ modeto a helical mode with winding number, l; FIG. 4B illustrates thehelical mode focused to a ring of light of radius R_(e)∝l with l=30; andFIG. 4C shows a multiply exposed photograph of a single colloidal silicasphere dispersed in water and circulating around the optical vortex ofFIG. 4B;

FIGS. 5A-1 and 5A-2 illustrate a phase holograph for encoding amicrofluidic gear pump using counter-rotating optical vortices withtopological charge l_(±)=±30; FIG. 5B illustrates the projected lightpattern and processing with the holograph before adaptive optimization;FIG. 5C shows after adaptive optimization; and FIG. 5D shows anoperating microfluidic pump filled with 700 nm diameter silica spheresand pumping water through a central channel at 5 μm/s;

FIG. 6 shows a general system for optical processing;

FIG. 7 shows a holographic optical trapping module;

FIG. 8 shows a holographic 3D imaging module;

FIG. 9 shows a white light based optical processing system; and

FIG. 10 shows a multipoint force monitoring/processing module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system constructed in accordance with an embodiment of the inventionis shown generally at 100 in FIG. 1. The system 100 uses acomputer-designed diffractive optical element (DOE) 110 to split asingle collimated laser beam 120 into multiple independent beams, eachof which subsequently is focused into an optical trap or tweezers 130(see FIG. 2C) preferably by a strongly converging objective lens 140 inFIG. 1. The DOE 110 preferably takes the form of a spatial lightmodulator (SLM) to create a computer-generated hologram (CGH) as shownin FIG. 2A. This CGH creates a plurality of light beams 125 in FIG. 2Bspecifically designed to create a particular pattern of the opticaltweezers 130 (see FIG. 2C). Projecting a sequence of computer-designedholograms with an SLM reconfigures the projected traps 130, therebytranslating trapped particles or manipulating objects along selectedindependent paths. The optical tweezer 130 is created in a conventionalmanner by bringing an intense beam of light, such as the laser beam 120,to a diffraction-limited focus. The light's electric field polarizesnearby dielectric objects; and the induced dipoles are drawn upintensity gradients toward the focus. Radiation pressure due toabsorption and surface scattering competes with this optical gradientforce and tends to repel an optically trapped particle. Stable trappingis only possible if the gradient force dominates. For this reason,optical tweezer systems often are built around the microscope objectivelens 140 (see FIG. 1) whose large numerical aperture and well-correctedaberrations optimize axial intensity gradients.

The optical trap 130 can be placed anywhere within the objective lens'focal volume by appropriately selecting the input beam's direction ofpropagation and degree of collimation. For example, a collimated beam135 passing straight into an infinity-corrected form of the objectivelens 140 comes to a focus in the center of the lens' focal plane, whileanother beam (not shown) entering at an angle comes to a focusproportionately off-center. A diverging beam focuses downstream of thefocal plane and a converging beam focuses upstream. By the same token,multiple beams entering the lens' input pupil simultaneously formmultiple ones of the optical trap 130 in the focal volume, each at alocation determined by its degree of collimation angle of incidence.

Using the system 100 of FIG. 1, FIG. 3A shows 173 colloidal silicaspheres arranged in a single plane within a three-dimensional samplevolume. Comparable planar rearrangements also can be implemented with aconventional single rapidly scanned optical tweezer in a time-sharedconfiguration. Unlike these other conventional techniques, however,holographic trapping also can create three-dimensional structures. Theimages in FIGS. 3B and 3C show the same spheres being reorganized intothe third dimension, their images changing appearance as they move alongan optical axis 145. Other three-dimensional optical trappingtechniques, such as the generalized phase contrast (GPC) method requiretwo-sided optical access and cannot stack objects along the optical axis145. By contrast, the system 100 can stack micrometer-scale objects atleast 12-14 deep along the optical axis 145.

In addition to arbitrary three-dimensional control, the holographictraps 130 offer other advantages over conventional scanned tweezers. HOTpatterns have extensive degrees of freedom than time-shared arrays whichmust periodically release and retrieve each trapped object.Additionally, the lower peak intensities required for continuouslyilluminated traps are less damaging to photosensitive samples.

A laboratory-scale implementation of a dynamic form of the holographicoptical tweezers 130 preferably used a Hamamatsu X8267 parallel-alignednematic SLM for the DOE 110 to reshape the laser beam 120 from afrequency-doubled diode-pumped Nd:YVO₄ laser (Coherent Verdi) into adesignated pattern of beams. Each is transferred to the entrance pupilof a 100×NA 1.4 oil immersion objective lens 140 mounted in a NikonTE2000U inverted optical microscope 150 and then focused into theoptical 130 trap. A dichroic mirror 160 reflects the laser beam 120 intothe objective lens 140 while allowing images of the trapped particles topass through to a video camera (not shown). When combined with a 0.63×widefield video eyepiece, this optical train offers a 86×65 μm² field ofview, with a magnification of 135 nm per pixel at the video camera.

The collimated laser beam 120 initially has planar wavefronts with auniform phase profile φ({right arrow over (ρ)})=0. The DOE 110 imprintsthem with the phase profile φ({right arrow over (ρ)}) at each 40 μm widepixel in a 768×768 array. The DOE's calibrated phase transfer functionimparts one hundred fifty distinct phase shifts ranging from 0 to 2π atthe operating wavelength of λ=532 nm. The phase shift imposed at eachpixel is specified through a computer interface with an effectiverefresh rate of 2 Hz for the entire array. Despite the DOE's inherentlylimited spatial bandwidth, it can project such sophisticated trappingpatterns as illustrated by considering FIGS. 2A-2C and 3A-3D.

Three-dimensional control is attained by integrating the objective lens'phase

$\begin{matrix}{{{\varphi_{z}\left( \overset{\rightarrow}{\rho} \right)} = {\pi\;\frac{\rho^{2}z}{\lambda\; f^{2}}}},} & (1)\end{matrix}$

profile, into the phase hologram, φ({right arrow over (ρ)}), encoding anarray of the optical traps 130. This translates the optical trap 130 byz along the optical axis 145 (see bottom edge view of FIG. 3C-2 takenfrom FIG. 3C-1). Here f is the focal length of the objective lens 140.The algorithms we have developed for computing trap-forming phaseholograms can separately specify φ({right arrow over (ρ)}) for eachtrap-forming beam, thereby allowing each of the optical traps 130 tomove independently with respect to the focal plane.

Engineering the individual traps' wavefronts imparts additionalfunctionality. For example, the helical phase profileφ_(l)({right arrow over (ρ)})=lθ mod 2π  (2)converts an ordinary Gaussian laser beam into a helical mode, and itscorresponding optical tweezer into a so-called optical vortex. Here θ isthe polar coordinate in the DOE plane, and the integer l describes thehelical wavefront pitch and is known as the topological charge.Destructive interference along the helical screw dislocation cancels thebeam's intensity along its axis all the way to the focus. As shown inFIG. 4B, optical vortices 170 thus focus to bright rings of light,rather than points. The image in FIG. 4B was obtained by placing amirror (not shown) in the microscope's focal plane and collecting thereflected light with the conventional imaging train. These dark form ofthe optical tweezers 130 have proved useful for trapping objects thatare not compatible with conventional optical tweezers 130, includingultra-high-index particles for photonic applications (such as assemblingthe icosahedral quasicrystal of FIGS. 3A-3C). These optical tweezers 130also can trap reflecting, absorbing and low-index particles that areuseful for biomedical applications.

Combining φ_(l)({right arrow over (ρ)}) with a phase hologram encodingan array of the optical tweezers 130 yields an array of optical vortices170 (see FIGS. 5A-1, 5A-2 and 5D). Our algorithms for computingphase-only hologram also can imprint vortex-forming helical phase frontsonto individual traps ones of the optical 130 in an array, creatingmixed patterns with different winding numbers, such as the array shownin FIG. 5B-5D.

Because of their helical phase profile, the optical vortices 170 carryorbital angular momentum, independent of polarization, that they cantransfer to trapped materials. The resulting torque can drive trappedobjects around the trap's circumference, as shown in FIG. 5D. Theoptical vortices 170 are useful, therefore, for creating motion at smalllength scales, for example in rotating semiconductor nanowires duringassembly. Arrays of the optical vortices 170 can act as pumps andmixers, which also are useful for preparing and processing materials forcharacterization and assembly. A typical pump is shown operating in FIG.5D. The optical vortices 170 can further act as conveyor belts for amanufacturing operation.

Holographic wavefront engineering lends itself to other advantageousapplications, with virtually any mode of light having potentialapplications. For example, the axicon phase profile φ_(γ)({right arrowover (ρ)})=γρ creates an approximation of a Bessel mode which focuses toan axial line trap whose length is controlled by γ. Arrays of Besselbeam line optical traps can pass through quite deep microfluidicchannels and thus improve optical fractionation of fluid-borne samples.

All of these trapping capabilities are controlled by the phase profileimprinted on the input laser beam 120 by the DOE 110 (preferably thespatial light modulator). Thus, they can be selected and changed in realtime under computer control, without requiring any hardwaremodifications. A single computer-designed phase hologram can projectdistinct patterns of the optical traps 130 in multiple wavelengths oflaser light. Multi-color trapping and photochemical processing is veryuseful for assembling multi-component tissues, materials and devices.

FIG. 6 illustrates a system 200 which includes modular subsystems thatcomprise the proposed integrated fabrication and characterizationsystem. Included in the system 200 can be holographic trapping subsystem210 and holographic microscopy subsystem 220. The primary assemblysystem 200 and the subsystems 210, 220 can be supported by variousspectroscopy components in both transmission and reflection, and anadvantageous form of force spectroscopy.

Rapid and precise fabrication of photosensitive materials requiresappropriate choice of wavelength. Earlier holographic trapping systemshave been based on high-powered visible lasers, both for their ease ofuse, and also for their comparatively high trapping efficiency formicrometer-scale objects. Longer wavelength lasers are less likely todamage biological materials, however, and can be more effective fortrapping semiconductor nanowires and related nanomaterials.

In the subsystem 210 of FIG. 6, recent advances in commercial fiberlasers make these a particularly attractive class of infrared sourcesfor trapping applications. The subsystem 210 preferably uses an IPGYLD-10-LP linearly polarized single-mode continuous wave fiber laser 240operating at 1075 nm. This laser 240 provides up to 10 W, which canpower up to 2000 independent traps. The laser beam 250 can be expandedto 5 mm diameter using a standard fiber-coupled beam expander (IPG) toilluminate the DOE 110 (preferably a spatial light modulator).

A preferred embodiment of the holographic optical trapping subsystem 210projects the optical traps 130 using a Holoeye HEO-1080p liquid crystalon silicon (LCoS) SLM tuned to provide 2π phase shift at the proposedlaser wavelength. This SLM (a preferred form of the DOE 110) comprisesan array of 1920×1080 phase pixels, each 8 μm across. In selecting asuitable SLM for a given application, several optimization criteria maybe considered. For example, larger numbers of phase pixels, higher ratesof phase pattern changes and finer resolution of phase modulation a1improves the performance of the holographic optical trapping subsystem.Smaller phase pixels allow for more compact design of the holographicoptical trapping subsystem 210.

In a most preferred embodiment, trapping and imaging will be based on achoice of Nikon CFI Plan Apo 100× and 60× oil immersion IR objectivelenses 140. These lenses 140 have proved excellent for optical trappingboth because of their high numerical aperture (NA 1.4), and also becauseof their excellent aberration correction. The IR variants of theselenses 140 are designed for operation at infrared and visiblewavelengths.

In a preferred embodiment shown in FIG. 7, laser power will be finetuned with a half-wave plate 300 in a precision rotation mount and apolarizing beam splitter 310. This also will improve diffractionefficiency by improving the beam's polarization. The beam splitter 310also provides a port for a second laser 320 at a different wavelength,useful for multicolor holographic trapping.

The role of the relay optics is to project an appropriately scaled imageof the DOE 110 face onto the input pupil of the objective lens 140. Thefield and depth of view for the trapping system then is set by thenumber of pixels spanning the projected aperture. With the 60× objectivelens 140, the proposed system 200 will be able to place traps atarbitrary locations over a 120 μm×120 μm area and at ±50 μm with respectto the focal plane. Three-dimensional placement resolution is roughly 30nm, and deliberately exploiting high-order diffraction will allow us toextend this range by nearly a factor of two, with limitations set byreduced diffraction efficiency.

As shown in FIG. 8, the optical train 400 that projects the holographictraps 130 is readily modified to work as an imaging system 410. As shownschematically in FIG. 8, light gathered by the objective lens 140 isprojected onto a video camera 420 by a relay lens 430 whose transferfunction is controlled by a second phase-only spatial light modulator440. The two physical relay lenses 430, 450 are arranged as a standard4f telescope, transferring the wavefronts of light emerging from theobjective lens' pupil to the face of the SLM 440. A virtual lensφ_(z)(ρ), encoded on the SLM 440 then selects the desired focal plane.

In the simplest implementation, the imaging train is focused to a planeat height z above the objective's focal plane by imprinting the samephase pattern, (see Eq. (1) used to displace the optical trap 130). Inthis case, the imaging plane can be effectively scanned through athree-dimensional sample without physically moving the sample relativeto the objective lens 140. The focusing hologram also can be used toadaptively correct for geometric aberrations in the imaging train.

Employing the same class of the SLM 440 for imaging and trapping ensuresthat the field of view and depth of focus for imaging will cover thesame range as the trapping system. This SLM's updates will besynchronized to the video camera 420 to gather volumetric data at videorates.

Images are most preferably acquired with a Roper Cascade 512Belectron-multiplied charge-coupled device (EMCCD) camera. This camera420 incorporates field-effect amplifiers at each pixel so that it canoffer both low-light level imaging and also extremely low-noisebright-field imaging. It also offers flexible triggering and electronicshuttering, as well as adjustable resolution and frame rate. Low-noiseoperation is particularly useful for particle tracking and deconvolutionmicroscopy, whose accuracy degrades rapidly with decreasingsignal-to-noise ratio.

Unwanted diffraction orders due to the SLM's pixelated structure can beeliminated with an appropriate set of pupils mounted with the ocularlens (not shown). Additionally, a small beam block 425 (shown in phantomin FIG. 8) introduced into the relay lens' intermediate focal plane canbe used to convert the optical train to dark-field operation. Otheramplitude- and phase-modifying elements introduced in this plane willprovide additional modes of operation, including variants ofphase-contrast microscopy.

Commercially available SLMs 440 impose different degrees of phasemodulation on light of different wavelengths. This means that theholographically focused system 410 would suffer from chromaticaberration were it used with white light illumination. This is aprincipal reason for replacing standard microscope illumination with amonochromatic source for bright-field and dark-field imaging. The system440 preferably is based on a 200 mW fiber-coupled laser diode 460,operating at 860 nm by SDL, Inc. Bending fiber 470 to scramble thewavefront yields uniform speckle-free illumination. The laser diode 460also can be rapidly gated for short exposures and multiple stroboscopicexposures.

The fiber-coupled laser 460 can be collimated with a commercial fibercollimator 480 and focused onto sample 465 with a second 60× objectivelens mounted as a condenser. The beam will be reflected into thecondenser 490 preferably using a dielectric multilayer mirror (ChromaTechnology) mounted at 45°, thereby providing additional optical accessto the condenser 490 at other wavelengths. In particular, thisarrangement will provide access for a standard white light illuminator,which can be convenient for some applications. In this case, the SLM 440will likely not play an active role, and the imaging system 410 willproduce standard two-dimensional images.

Volumetric image reconstruction can best be performed using standardalgorithms of deconvolution microscopy implemented in the IDLprogramming language. Simple deblurring operations with thenearest-neighbor algorithm can proceed in near-real time on a standardworkstation. This will be ideal for on-line inspection of systems beingassembled through holographic trapping. More accurate reconstructionswill proceed either with myopic deconvolution or with constrainedmaximum entropy algorithms, depending on the nature of the sample. Thesemore computationally intensive algorithms are useful for quantitativestructural measurements on finished objects.

The condenser and illumination system 410 can be mounted on aspring-loaded rack-and-pinion translation stage both to facilitateKöhler alignment and also to provide access to samples.

The samples 465 can be mounted on an integrated translation stage withthree-axis control. Coarse focusing is performed with a precisionspring-loaded rack-and-pinion drive to minimize drift. Precisecomputer-controlled focusing and lateral translation can be performedwith a conventional Mad City Labs Nano-View LP200, which combinesstepping-motor two-axis coarse translators with piezoelectric three-axiscontrollers. The fine controllers offer 200 μm range in each axis with0.4 nm resolution and better than 1 nm repeatability.

A major application area for the proposed fabrication instrument is inholographic assembly of photonic materials and devices. Another involvesorganization and monitoring of living biological samples. Consequently,a preferred system 500 shown in FIG. 9 incorporates a fiber spectrometer510 and a pair of white light sources 520 to provide transmission andreflection spectra in real time. Spectroscopic information then can beused to fine tune structures before they are permanently set in place.It also can be used to assess trapped cells' viability and theirresponse to external stimuli. For optoelectronic applications, thespectroscopy subsystem 530 can be used to analyze samples' fluorescencewhen exposed to the trapping laser. Some semiconductor nanowires areobserved to fluoresce brightly when trapped, and this fluorescence canbe useful for selecting nanowires for assembly.

The spectroscopy subsystem 500 most preferably includes an Ocean OpticsUSB4000 fiber optic spectrometer, which offers better than 4 nmwavelength resolution over the range 300 to 1000 nm. Some regions inthis wavelength range will be suppressed by the transmissioncharacteristics of the dielectric multilayer mirrors used in theholographic trapping and imaging train. Mounting the spectrometer'sinput coupler below the microscope permits simultaneous holographictrapping, three-dimensional imaging and real-time spectroscopy, however.This type of coordination is essential for creating precise photonicstructures under interactive control and also will be useful forcharacterizing biological systems during optical micromanipulation. Theloss of sensitivity in selected wavelength ranges, therefore, iscompensated by the additional functionality. Two Ocean Optics R-LS-1-LLrack-mounted halogen light sources will provide broad-band illuminationfor transmission and reflection spectroscopy. The entire system can becalibrated with standard samples.

In another form of the invention, multi-point force spectroscopy andmanipulation can be performed using a calibrated arrangement of opticaltweezers' potential energy wells. An object's instantaneous displacementfrom the trap's equilibrium point can be used to measure theinstantaneous force making it move. Most effectively, the potentialenergy well of a single one of the optical tweezers 130 can becalibrated by tracking the thermally driven motions of a trappedparticle. This general approach avoids the necessity of characterizingand calibrating an externally applied reference force. Statisticallyoptimal methods can be used for analyzing trajectories of opticallytrapped particles to obtain time-resolved measurements of the forces onmultiple holographically trapped particles simultaneously. These methodscan be applied also to video microscopy data obtained in the proposedsystem. Imaging measurements of forces, however, only work when thetrapped particles' displacements are large enough and slow enough totrack with a video camera. They also require accurate calibrations foreach one of the optical traps 130.

Light scattered out of the optical trap or tweezer 130 by a trappedparticle interferes with the unscattered portion of the beam to yield aninterference pattern in the far-field forward-scattering direction. Thisinterference transforms small particle motions into large intensityvariations. Measuring these variations with a quadrant photodioderecords the particle's displacement with sub-nanometer resolution over abandwidth of tens of kilohertz. Once translated into equivalent forces,this technique can attain attonewton force resolution and can measureforces as large as several piconewtons. These specifications greatlyexceed what is possible through imaging-based measurements.

It is noted that an alternative technique has been developed thatrelaxes the requirement to calibrate the optical tweezer 130 for forcemeasurements and also is amenable to parallelization. In a furtherembodiment of the invention, the trapped particle deflects the trappingbeam by an amount that depends on its displacement from the center ofthe trap. The beam's mean deflection corresponds to an average change inmomentum imparted to the trap's photons by the particle, and thus to aforce when normalized by the flux of photons in the beam. The effectiveforce deflecting the beam equals the force displacing the particle byNewton's third law. A single calibration of the trapping beam'sintensity and the imaging train's magnification therefore calibrates theforce transducer, independent of the optical tweezer's trappingcharacteristics.

An individual trapping beam's displacement can therefore be measured byimaging the far-field scattering pattern through the condenser in aplane intermediate between the objective's front and back focal planes.Thus, optical deflection force spectroscopy can be applied to multipleoptical traps 130 simultaneously, provided that the traps' images areresolved in the intermediate plane. The individual traps' deflectionscan be measured with sufficient precision with a conventional videocamera 600 (see FIG. 10) to provide sub-femtonewton resolution over aten piconewton range at a bandwidth limited by the camera's frame rate.The benefit over previously described approaches is that holographictrapping force spectroscopy provides information from a large number oftraps simultaneously. The implementation then uses the video camera 600,protected with neutral density filters to detect the holographic traps'forward scattered beams 610. The camera 600 is focused with a tube lens620 to optimize the trade-off between force resolution and spatialresolution. Output from the camera 600 will be digitized and analyzedwith a conventional analysis software of the Applicant.

In yet another aspect of the invention, raw materials can be introducedto the optical fabrication system and finished products removed usingmicrofluidic sample handling. Pioneered with conventionalmicrolithography, microfluidics systems have since been implemented inpolymeric materials with soft lithographic techniques that permit rapidprototyping at extremely low costs. Polymer-based microfluidic systemsalso permit integration of microscopic pumps and valves. Pulsating flowsfrom such pumps can be compensated by phase-locked modulation in laserintensity to maintain optimal conditions for optical fractionation.Integrated microfluidic systems also are compatible with electrokineticdriving technologies developed for capillary electrophoresis.

In still another embodiment of the invention, a single opticalprocessing instrument can be combined with a large number of distinctmicrofluidic chips to generate a range of different optical fabricationand fractionation applications, such as manufacturing microfluidic chipsthrough soft lithography in polydimethysiloxane (PDMS).

Another embodiment of the invention enables manufacture and qualityassurance in fabricating three-dimensional structures out of dielectricbuilding blocks. One application is to assemble and characterizethree-dimensional photonic bandgap materials. These “semiconductors forlight” have been demonstrated for radio and microwave wavelengths usingmacroscopic assembly techniques, and have been demonstrated for visiblewavelengths in one and two dimensional lithographically definedmicrostructures. Processing high-index materials into three-dimensionalphotonic bandgap microstructures for optical applications has provedchallenging. Creating appropriate small-scale structures with theproposed optical fabrication instrument therefore would open up newavenues for research and development in photonics as well as inbiomolecular spectroscopy.

Very recently, icosahedral quasicrystals have been identified as thebest candidate structure for achieving three-dimensional photonicbandgaps. These pose even greater challenges to conventional fabricationtechniques than periodic structures. These structures can beholographically assembled and typical results appear in FIG. 2C. Thesecolloidal quasicrystals were assembled from silica spheres and thengelled into solid structures by photopolymerizing the surrounding fluidmedium.

We also can use the optical fabrication and characterization instrumentto organize ultra-high-index titania spheres into comparable permanentstructures, and to measure their transmission and reflection spectra atoptical wavelengths. Three-dimensional holographic microscopy will beparticularly important for guiding and assessing the assembly process.Bulk photopolymerization of prototype structures can take advantage ofultraviolet light-emitting diodes (LED's) arranged as a ring illuminatoraround the condenser lens. Properties of the gel can be assessed withforce spectroscopy on the spheres themselves.

In yet another application of the invention the optical tweezers 130 canmanipulate and process semiconductor nanowires into three-dimensionalstructures to enable creating electronic and optoelectronic devices fromchemically nanostructured materials. Heretofore, nanowire devices werecreated by randomly depositing the wires onto substrates and thendefining functional structures through painstaking lithographictechniques. Now, devices can be assembled to order and use the systemsdescribed herein to build functional devices out of silicon nanowires,with a particular emphasis on sensor applications for biological andenvironmental monitoring.

In yet another embodiment, infrared holographic trapping is useful formanipulating living biological cells. Holographic trapping, inparticular, is useful for arranging multiple disparate cells intospecific three-dimensional configurations. This kind of structuring iscrucial for the proper growth and development of cells in livingtissues. Optically organized cellular assemblies have been demonstratedin model systems, including hepatocytes as a liver progenitor and isletcells for creating pancreatic implants. The three-dimensional cellularassemblies are transformed into artificial tissues by synthesizing abiodegradable gel around them.

Holographic trapping coupled with holographic microscopy withinmicrofluidic environments will greatly facilitate and accelerate theoptical assembly of artificial tissues. For example, the systems hereincan be used to organize chondrocytes and osteoblasts intothree-dimensional models for developing teeth, with the intention ofcreating transplantable artificial dentin.

The foregoing description of embodiments of the present invention havebeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments, and with various modifications, as aresuited to the particular use contemplated.

1. An optical imaging system for examining an object comprising: a lightsource to produce a light beam; a spatial light modulator; an opticaltrain for processing the light beam to produce a plurality of opticaltraps; the optical train including an objective lens and a relay lenswith a transfer function provided by the spatial light modulatorgenerating a phase only hologram thereby providing an imaging trainfocused to a plane at height z above an objective lens' focal plane byimprinting a same hologram phase pattern used to form the optical traps,thereby enabling an imaging plane to be effectively scanned through theobject in three dimensions without moving the object relative to theobjective lens.
 2. The optical system as defined in claim 1 wherein thephase only hologram operates to correct geometric aberrations in theoptical train.
 3. The optical system as defined in claim 1 furtherincluding a video camera which is synchronized to the spatial lightmodulator to gather volumetric data about the object at video rates. 4.The optical system as defined in claim 1 further including a beam blockdisposed into an intermediate focal plane of the relay lens therebyconverting the optical train to dark field operation.
 5. The opticalsystem as defined in claim 1 further including a white light source forthe light source, a dielectric multi-layer mirror and a second objectivelens positioned operatively as a condenser, thereby enabling white lightimaging of the object.
 6. The optical system as defined in claim 1further including a fiber spectrometer and the light source comprisingtwo white light sources to provide transmission and reflection spectrain real time, thereby enabling fine tuning of structures associated withthe object prior to fixing the structures.
 7. The optical system asdefined in claim 2 further including an optical subsystem disposed belowthe imaging system to provide simultaneous trapping by the opticaltraps, imaging and real-time spectroscopy.